1. Field of the Invention
The present invention relates to a liquid ejection head and an image forming apparatus, and more particularly, to a liquid ejection head which generates a minute liquid droplet from a liquid surface by means of a surface acoustic wave, and to an image forming apparatus which uses such a liquid ejection head.
2. Description of the Related Art
In the related art, Japanese Patent Application Publication No. 2000-62161 and Japanese Patent Application Publication No. 9-267473 disclose inkjet heads based on a method which ejects ink by emitting a surface acoustic wave (SAW) into ink. In a general surface acoustic wave type of head of this kind, when a surface acoustic wave (SAW, Rayleigh wave) is caused on the surface of a solid and this wave is introduced into a liquid by leaking, then either a group of minute liquid droplets are drawn off from the vicinity of the three-phase interface by a longitudinal wave radiating in a direction forming an angle (Rayleigh angle) with respect to the normal of the incident axis, or alternatively, a group of minute liquid droplets is ejected on the basis of a mechanism in which a capillary wave is indirectly caused in the liquid by means of the radiation of the aforementioned longitudinal wave, this capillary wave is amplified, and the liquid particles are formed from the wave crests when the capillary wave reaches a certain amplitude (formation of a so-called “capillary mist”).
However, in the former case (where a group of minute liquid droplets are drawn off from the vicinity of the three-phase interface), there is difficulty in controlling the ejection direction, and there is also variation in the diameter of the particles. On the other hand, the latter case (based on the formation of a so-called capillary mist) uses a mechanism which amplifies the capillary wave on the liquid surface by parametric resonance, and therefore it is not suitable for high-viscosity liquids. Moreover, there is an inherent and fundamental restriction in that, when energy satisfying the resonance conditions is applied continuously for a prescribed period of time, then a phase transition occurs due to viscous heat generation before the amplitude increases to a level at which particles are generated.